This invention relates to security alarm systems which give an alarm when the power line of electrical equipment is disconnected and more particularly, to a power receptacle which accomodates the electrical equipment power line plug and which is equipped to provide an alarm signal in response to the occurance of certain conditions.
Heretofore, it has been proposed to provide a power receptacle into which a conventional power plug from electrical equipment can be inserted, with an alarm system which initiates an alarm signal when the power plug is disconnected from the receptacle. Some of these systems are used for security to provide an alarm when electrical apparatus such as a television set is unplugged in order to steal it. In some security alarm systems of this sort, a special line is provided from the location of the receptacle to a remote observer location where the observer is signalled when the electrical apparatus is unplugged. In other such systems, an alarm signal generated at the receptacle is transmitted over the AC power line to a remote location for signalling an observer.
In many of the prior systems, the plug from the electrical apparatus is not of conventional design and neither is the receptacle. Hence, only apparatus equipped with the special plug can be secured by the alarm system. Also, in some of the prior systems, a dummy plug can be inserted in the receptacle after removal of the plug from the apparatus, and upon insertion of the dummy plug, the alarm is turned off. In addition, many of these prior systems will either become inoperative or they will give a false alarm when the AC power at the receptacle is turned off. Furthermore, in these prior systems, the receptacle can be tampered with and made inoperative so that no alarm is given when the apparatus is unplugged from the receptacle. None of the prior systems provide an alarm when the power line from the plug to the electrical apparatus is cut or short circuited to ground, and so a thief need only cut that line and remove the apparatus without fear of setting off the alarm.
In a conventional AC power system, a short circuit to ground instantly draws a high current from the AC power. The conventional safeguard against this is a fuse or circuit breaker at the AC power junction box that disconnects power to all receptacles and electrical items on the AC power line that are energized through that fuse or circuit breaker. This protection would seem to be excessive. It should only be necessary to disconnect power to the receptacle load or electrical item that is short circuited.